It is generally known in the art, as taught in WO 9102039, EP 0532831A and EP 0802251 A, that block copolymers comprising terminal poly(styrene) blocks and one or more central poly(isoprene) blocks, more particularly triblock copolymers, are used in hot melt adhesive compositions for the manufacture of disposable articles. More specifically triblock copolymers having a poly(styrene) content of from 25 to 35 wt % and having a total apparent molecular weight of from 140,000 to 145,000 (e.g., KRATON® D-1165 block copolymer) are used in hot-melt adhesive compositions in the diaper industry in two manners:
1. as assembly adhesive to glue the diaper poly(ethylene) main frame and
2. as an attachment adhesive to glue the elastic attachments which are used as waist and leg bands.
Such adhesives must be lightly colored, transparent, low in odor, sprayable at 350° F., show good adhesion to poly(olefin) films and not strike through the non-woven back sheet. They must also be relatively inexpensive.
Elastic attachment adhesive is used to adhere elastic threads to polyethylene and non-woven films in order to form an elastic waist or leg band. Multiple strands of elastic threads are elongated, commonly 300%, and coated with adhesive composition at a temperature of about 300° F. just prior to being pinched inside of the poly(olefin) and non-woven webs. Like construction adhesives, they are usually sprayed in a spiral pattern. When the ends of the elastic are cut later in the process, the composite contracts causing the non-woven and poly(olefin) films to pucker to form an elastic band.
Although block copolymers such as KRATON D-1155 block copolymer show an acceptable hot-melt viscosity/temperature profile and have a sufficiently low viscosity at the usually applied hot-melt temperatures of up to 320° F. thereby enabling efficient processing, there is still an economic need for improved block copolymers which demonstrate improved processing efficiency, in combination with other attractive properties noted hereinbefore.
It will be appreciated that a relatively low hot-melt viscosity facilitates a high production speed in a diaper manufacturing line without the production of significant amounts of off spec product. Problems which normally cause such off spec product may be, for example, adhesive viscosity degradation, adhesive discoloration, damage to the polyethylene film or formation or char. Accordingly, developmental efforts in diaper manufacture continue today.
As a result of extensive research and experimentation it has now surprisingly been found that block copolymers comprising terminal, predominantly poly(styrene) blocks and central (B/S) blocks, afford a good sprayable hot-melt viscosity at the temperatures preferably applied in the diaper industry, i.e. in the range of from about 250 to about 350° F. In addition, these block copolymers afford excellent color stability, superior adhesive performance and better viscosity stability compared to conventional adhesive compositions based on S-I-S block copolymer.